The invention relates to a magnetic resonance imaging method. In order to form a magnetic resonance image of an object, the object is arranged in a steady, as uniform as possible magnetic field. Often only a part of the object is imaged; to this end, the part of the object to be imaged is then arranged in the steady magnetic field. The steady magnetic field orients spins in the object to be examined predominantly in the direction of the steady magnetic field. According to such a magnetic resonance imaging method, spins in an object to be examined are excited. Relaxation of the excited spins produces magnetic resonance signals which are acquired. A magnetic resonance image is reconstructed from the magnetic resonance signals acquired.
A magnetic resonance imaging method of this kind is known from U.S. Pat. No. 5,378,987.
The known magnetic resonance imaging method is dedicated notably to measurement, on the basis of the magnetic resonance signals, of a temperature distribution in the object to be examined. The cited United States patent deals with the problems caused by displacements of the object to be examined. The cited United States patent mentions notably that the measured temperature distribution may be spoiled by displacement of the object to be examined. The known magnetic resonance imaging method offers a rather cumbersome, time-consuming solution to this problem. The known magnetic resonance imaging method notably necessitates the execution of separate magnetic resonance excitation sequences for the detection of displacements of the object and for the measurement of the frequency shift due to variation of the temperature, referred to as xe2x80x9cchemical shift dataxe2x80x9d, respectively. According to the known magnetic resonance imaging method, such magnetic excitation sequences must both be repeated for different values of the echo time in the measurement of the chemical shift.
It is an object of the invention to provide a magnetic resonance imaging method wherein it is comparatively simply achieved that hardly any disturbances occur due to motions of the object to be examined.
This object is achieved by means of a magnetic resonance imaging method according to the invention wherein
magnetic resonance signals are acquired,
the position of a measuring site is determined, and
the magnetic resonance image is reconstructed from the magnetic resonance signals and on the basis of the position of the measuring site.
In accordance with the invention, the position of a measuring site is separately determined, such a position can be separately measured. Furthermore, a predetermined geometrical relationship exists between the measuring site and the region reproduced in the magnetic resonance image. On the basis of the position determined for the measuring site, disturbances due to motion of the object to be examined can be avoided in the magnetic resonance image on the basis of the predetermined geometrical relationship between the measuring site and the region to be imaged (FIG. 3 step 320). The object to be examined is a patient. During the acquisition of the magnetic resonance signals, the patient is liable to move and/or motions are liable to occur within the body of the patient, due to the respiration and/or the heartbeat. The magnetic resonance imaging method according to the invention notably ensures that hardly any disturbances which are due to such motions in and/or of the patient occur in the magnetic resonance image.
The invention is implemented in such a manner that a selected slice of the object to be imaged contains the measuring site. Notably when images of the same slice are repeatedly formed it is achieved that the same slice is always accurately reproduced. It is then ensured that the selected slice always extends through the measuring site. The selection of such a slice is performed on the basis of an RF excitation in combination with a selection gradient. Such a selection gradient is superposed on the steady magnetic field.
The magnetic resonance image can also be accurately corrected for motion in and/or of the object on the basis of the measured position of the measuring site and the predetermined geometrical relationship between the measuring site and the region to be imaged.
It has been found that the position of the measuring site can be readily determined. As a result, disturbances in the magnetic resonance image which are due to motion can be very simply counteracted.
These and other aspects of the invention will be elaborated on the basis of the following embodiments which are defined in the dependent claims.
Preferably, a clearly recognizable detail of the object to be examined and an indication of the measuring site are reproduced in the magnetic resonance image. This is realized by reproducing the relevant detail and the measuring site together in the magnetic resonance image (FIG. 3 step 380). On the basis of the predetermined geometrical relationship between the measuring site and the relevant detail, the correct position of the reproduction of the detail relative to the indication of the measuring site in the magnetic resonance image can also be derived (FIG. 3 step 390). On the basis of the derived correct position of the detail it can then be readily checked whether the position of the detail has shifted due to motion in and/or of the object and, if desired, the position of the detail in the magnetic resonance image can be corrected.
The magnetic resonance imaging method according to the invention is particularly suitable for accurately deriving the local temperature distribution in the object to be examined by means of the magnetic resonance imaging method. To this end, reference magnetic resonance signals are first acquired at a predetermined reference temperature, after which measuring magnetic resonance signals are acquired at a locally increased temperature in the object to be examined. A reference magnetic resonance image of the part of the object to be examined is reconstructed from the reference magnetic resonance signals. A measuring magnetic resonance image of the part of the object to be examined is reconstructed from the measuring magnetic resonance signals for which the temperature has locally been varied (FIG. 3 step 370). The temperature variation causes a frequency shift of the measuring magnetic resonance signals relative to the reference magnetic resonance signals; this frequency shift will be referred to as xe2x80x9ctemperature dependent chemical shiftxe2x80x9d. The measuring site is reproduced in the reference magnetic resonance image as well as in the measuring magnetic resonance image and the position of the measuring site is separately reproduced so as to be suitably recognizable in the reference magnetic resonance image and the measuring magnetic resonance image, or is separately measured (FIG. 3 step 380). Furthermore, the predetermined geometrical relationship between the reproduction of the detail and the indication of the measuring site in the reference magnetic resonance image is also determined on the basis of the reference magnetic resonance image. As a result, the measuring magnetic resonance image and the reference magnetic resonance image can be made to register, the same details in both images then being situated in the same position in the images relative to the indication of the measuring site in both images. It is thus achieved that the local temperature variation can be accurately derived from the frequency shifts of the measuring magnetic resonance signals relative to the reference magnetic resonance signals while avoiding disturbances due to motion. The determination of the local variation of the temperature on the basis of the temperature dependent chemical shift (FIG. 3 step 350) per se is rather sensitive to motion, because the measuring magnetic resonance signals are spatially encoded on the basis of the frequencies of these signals. Because a separate determination or measurement of the position of the measuring site is available according to the invention, the effect of the temperature dependent chemical shift can be separated from the frequency encoding of the position in space whereto the magnetic resonance signals relate.
The registration of the measuring magnetic resonance image with the reference magnetic resonance image, will be better as the positions of more different details in the measuring magnetic resonance image are corrected.
The measuring magnetic resonance image is preferably made to register with the reference magnetic resonance image by counteracting disturbances due to motions during the formation of the measuring magnetic resonance image. Disturbances due to motion can be counteracted according to the invention by ensuring, on the basis of the position determined for the measuring site, that the reference magnetic resonance signals and the measuring magnetic resonance signals relate to or originate from the same region of the object to be examined. This can be readily achieved by selecting, on the basis of the measuring site, the same slice of the object for the acquisition of the reference magnetic resonance signals as well as for the acquisition of the measuring magnetic resonance signals. Thus, prior to the reconstruction of the reference magnetic resonance image and the measuring magnetic resonance image it is already ensured that these two magnetic resonance images register.
It is also possible to make the reference magnetic resonance image and the measuring magnetic resonance image register after the reconstruction from the reference magnetic resonance signals and the measuring magnetic resonance signals. The measuring site is preferably chosen to be such that the indication of the measuring site is situated in substantially the same positions in the reference magnetic resonance image and the measuring magnetic resonance image. The shift of the reproduction of the detail in the measuring magnetic resonance image relative to the reproduction of the same detail in the reference magnetic resonance image then follows from the relative position of the reproduction of the same detail in the reference magnetic resonance image and the measuring magnetic resonance image relative to the indication of the measuring site.
It is a further object of the invention to provide a magnetic resonance imaging method enabling accurate measurement of the temperature distribution in the object to be examined.
This object is achieved by means of a method of forming a magnetic resonance image wherein:
magnetic resonance signals are acquired,
the position of a measuring site is measured, and
the temperature at the measuring site is derived from the magnetic resonance signals. Because the position of the measuring site is separately measured, the effect of the frequency shift of the magnetic resonance signals (the temperature dependent chemical shift), caused by the temperature variation, can be separated from the frequency encoding of the spatial positions whereto the magnetic resonance signals relate. It is notably possible to derive the local temperature at the exact position of the measuring site from the magnetic resonance signals. The effect of motion of and/or in the object to be imaged, i.e. the patient to be examined, is notably reduced.
Preferably, a set of reference magnetic resonance signals is acquired at a predetermined reference temperature (FIG. 3 step 330). When the local temperature within the body of the patient to be examined is derived by means of the method according to the invention, the reference temperature is the body temperature of the patient to be examined. Subsequently, the temperature is locally increased and a set of measuring magnetic resonance signals is acquired at the increased temperatures (FIG. 3 step 340). Because the position of the measuring site has been separately measured, the temperature dependent chemical shift can be derived from the frequency shift of the measuring magnetic resonance signals relative to the reference magnetic resonance signals, (FIG. 3 step 350) so that the local temperature can be determined at the measuring site (FIG. 3 step 350). Because the position of the measuring site has been separately measured, the accuracy of the determination of the temperature will hardly be affected when motion of and/or in the patient to be examined occurs between the acquisition of the reference magnetic resonance signals and the measuring magnetic resonance signals. Furthermore, the position of the measuring site at which the local temperature increase is measured is particularly reliable and notably is hardly affected by motions of and/or in the patient to be examined.
Further advantages are achieved by deriving the temperature distribution in the object to be examined on the basis of the measuring magnetic resonance signals, the reference magnetic resonance signals and the position of the measuring site determined. Preferably, this temperature distribution is reproduced as a thermal image. Brightness or color values represent the local temperature in such a thermal image. Furthermore, such a thermal image also contains image information concerning the anatomy of the patient. This image information is acquired by means of magnetic resonance imaging methods which are known per se. Such a temperature distribution constitutes a useful technical aid notably for performing thermal treatment on the body of the patient. Such thermal treatments concern (laser) ablation of tissue. Laser radiation is then used to destroy diseased tissue by local heating. The diseased tissue in the desired region can be readily locally thermally treated on the basis of the temperature distribution reproduced in the thermal image.
It has been found that a microcoil is particularly suitable for determining the position of the measuring site. The microcoil is introduced into the body of the patient (FIG 3 step 300). The microcoil receives magnetic resonance signals practically exclusively from the immediate vicinity of the microcoil (FIG. 3 step 310). The magnetic resonance signals received by the microcoil thus accurately represent the current position of the microcoil. The location where the microcoil is situated thus constitutes the measuring site. Microcoils for interventional e.g. endocavitary applications are smaller than approximately 1 cm. Generally speaking, microcoils having dimensions of between 0.5 mm and 3 mm are used, but even smaller microcoils, being smaller than 1 mm or even as small as approximately 0.1 mm, are also used to determine the position of the measuring site particularly accurately. The microcoil is preferably used in conjunction with an energy-dissipating element. Such an energy-dissipating element locally deposits energy in the form of laser radiation, in the tissue so as to increase the local temperature. The microcoil is preferably arranged near the energy-dissipating element. Furthermore, the microcoil is advantageously used in combination with a temperature sensor. Use is preferably made of a temperature sensor in the form of a fiber thermometer. Such a fiber thermometer has hardly any disturbing effect on the magnetic resonance signals. Preferably, the temperature sensor is arranged in the immediate vicinity of the microcoil. This enables separate measurement of the temperature in the direct vicinity of the microcoil. The temperature distribution relative to the temperature measured at the measuring site can be derived on the basis of the position of the measuring site as determined by the microcoil and the temperature at the measuring site as determined by the temperature sensor.
The invention also relates to a magnetic resonance imaging system. The magnetic resonance imaging system according to the invention is arranged to determine the position of the measuring site. Preferably, the magnetic resonance imaging system according to the invention is provided with the microcoil. Using the microcoil, magnetic resonance signals representing the position of the measuring site are acquired at the area of the measuring site or in the immediate vicinity of the measuring site. Such a microcoil enables measurement of the position of the measuring site with an accuracy of less than 1 mm and even 0.1 mm. This accuracy is dependent inter alia on the accuracy of measurement of the temperature and the phase of the position magnetic resonance signals. It is also advantageous to use a plurality of microcoils, preferably two or three microcoils. When two microcoils are used, the position and the direction of the line through the microcoils can be measured; when three microcoils are used (not in one line), the position and the orientation of the plane through the three microcoils can be measured (FIG. 3 step 320). It is also possible to use an even larger number of microcoils in order to measure deformations in the anatomy of the patient. The magnetic resonance image can be corrected for the measured deformation by image processing on the basis of the measured deformations.
The invention also relates to a computer program. The computer program according to the invention contains instructions for the acquisition of magnetic resonance signals, for the determination of the position of the measuring site, and for the reconstruction of the magnetic resonance image from the magnetic resonance signals on the basis of the position of the measuring site determined. The magnetic resonance imaging system includes a computer for executing the various functions of the magnetic resonance imaging system. When the computer program according to the invention is loaded into the computer of the magnetic resonance imaging system, the method according to the invention can be carried out by means of the magnetic resonance imaging system.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment described hereinafter and the accompanying drawing; therein: